1. Field of the Invention
The present invention relates to a pick-up head for reading out signals recorded on an optical information storage disk, and more particularly to such a pick-up head employing an optical waveguide.
2. Description of the Prior Art
Optical information storage disks (hereinafter referred to as "optical disks") are widely used as mediums for recording image signals, audio signals, and the like thereon. The signals recorded as pits on the optical disk are read out by a pick-up head having an optical lens system. The pick-up head emits a light beam such as a laser beam that is directed to the information storage surface of the optical disk, and detects the level of light reflected from the optical disk to ascertain whether there is a pit or not at the location which is hit by the light beam.
One proposed pick-up head for use with optical disks employs an optical waveguide (see Electronic Communications Society Technical Report OQE 84-109, page 97). This pick-up head comprises an optical waveguide made of a material capable of propagating a surface elastic wave, a light source for emitting light into the optical waveguide, a waveguide lens for converting the wave light traveling through the optical waveguide into parallel-ray wave light, a focusing grating coupler formed on a surface of the optical waveguide for guiding the wave light out of the optical waveguide and focusing the wave light onto the surface of an optical disk, and a light detector for detecting the light reflected by the surface of the optical disk back into the optical waveguide through the focusing grating coupler. The pick-up head of this construction is lighter, smaller, and easier to manufacture than the conventional optical pick-up heads. Like the conventional pick-up head employing the optical lens system, the pick-up head with the optical waveguide also requires that the light beam be directed precisely to a desired track containing a series of pits (tracking control), and that the light beam be focused precisely on the surface of the optical disk (focusing control).
Heretofore, the focusing control has been effected by using a focusing coil or the like to move the optical lens system of the pick-up head along its optical axis, and the tracking control has been effected by using a tracking coil or the like to move the optical lens system in a direction normal to the optical axis. Such tracking and focusing control is also effective in tracking and focusing the pick-up head with the optical waveguide. Use of the above electromagnetic or mechanical means for the tracking and focusing control of the optical waveguide pick-up head is however disadvantageous in that the pick-up head inevitably becomes large in size, partly offsetting the benefit derived from incorporation of the optical waveguide.